Dark matter proponents theorize that most of the known Universe is actually made of material that doesn’t interact with light, making it invisible and undetectable, but that this material accounts for much of the gravitational pull among galaxies. A counter explanation introduced by the Israeli physicist Mordehai Milgrom in 1983, the MOdified Newtonian Dynamics (MOND) theory, says this gravitational pull exists because the rules of gravity are slightly altered. Instead of attributing the excess gravitational pull to an unseen, undetectable dark matter, MOND suggests that gravity at low accelerations is stronger than would be predicted by a pure Newtonian understanding. In addition, MOND makes a bold prediction: the internal motions of an object in the cosmos should not only depend on the mass of the object itself, but also the gravitational pull from all other masses in the Universe — the so-called external field effect. Now, astrophysicists from South Korea, the United Kingdom and the United States report a detection of this external field effect in 153 disk galaxies from the Spitzer Photometry and Accurate Rotation Curves (SPARC) database.
Messier 63 (also known as M63, NGC 5055 and the Sunflower Galaxy), a spiral galaxy located approximately 29.5 million light-years in the constellation of Canes Venatici, has the strongest external field among SPARC galaxies. Image credit: NASA / ESA / Hubble.
“The external field effect is a unique signature of MOND that does not occur in Newton-Einstein gravity,” said co-author Dr. Stacy McGaugh, a researcher in the Department of Astronomy at Case Western Reserve University.
“This has no analogy in conventional theory with dark matter. Detection of this effect is a real head-scratcher.”
“I have been working under the hypothesis that dark matter exists, so this result really surprised me,” added lead author Dr. Kyu-Hyun Chae, a researcher in the Department of Physics and Astronomy at Sejong University.
“Initially, I was reluctant to interpret our own results in favor of MOND. But now I cannot deny the fact that the results as they stand clearly support MOND rather than the dark matter hypothesis.”
The scientists analyzed 153 rotation curves of disk galaxies from the SPARC sample.
They deduced the external field effect by observing that galaxies in strong external fields slowed, or exhibited declining rotation curves, more frequently than galaxies in weaker external fields, as predicted by MOND alone.
“I was skeptical by the results at first because the external field effect on rotation curves is expected to be very tiny,” Dr. McGaugh said.
“We spent months checking various systematics. In the end, it became clear we had a real, solid detection.”
“Skepticism is part of the scientific process and understands the reluctance of many scientists to consider MOND as a possibility,” she added.
“I came from the same place as those in dark matter community. It hurts to think that we could be so wrong. But Milgrom predicted this over 30 years ago with MOND. No other theory anticipated the observed behavior.”
The team’s paper was published in the Astrophysical Journal.
Kyu-Hyun Chae et al. 2020. Testing the Strong Equivalence Principle: Detection of the External Field Effect in Rotationally Supported Galaxies. ApJ 904, 51; doi: 10.3847/1538-4357/abbb96
This article is based on text provided by Case Western Reserve University.